The harvesting and utilization of clean and renewable energy, such as energy from solar and wind, have experienced a rapid evolution. Implementation of these intermittent energy resources requires large-scale energy storage systems to store and regulate the power output among peak and off-peak hours. As the most popular electrochemical energy storage device, lithium ion batteries (LIBs) are considered to be the most promising candidate due to their high energy density. However, in such large-scale applications, cost, lifetime and safety are particularly important factors to be considered.[1] Compared to expensive and flammable non-aqueous LIBs, aqueous batteries with water-based electrolyte possess a natural advantage in these areas. Furthermore, they do not require strict oxygen- and water-controlled manufacturing environments and thus have much lower fabrication costs.
The development of aqueous battery systems has progressed rapidly in recent years, including monovalent Li+, Na+ and K+ and divalent Mg2+ and Zn2− systems. [2] Among them, aqueous rechargeable zinc ion batteries (ARZIBs) have attracted much attention due to the low price, rich global distribution, high stability, relatively low redox potential, and high theoretic capacity (825 mAh g−1) of zinc metal. These merits of ARZIBs have substantially raised their application potential in large-scale energy storage systems and even in electric vehicles. Most recently, α-MnO2, β-MnO2 and Zn0.25V2O5.nH2O nanofibers have been applied to ARZIBs.[2e-2g]
Although ARZIBs have been the focus of recent research, the lack of suitable cathode materials is a significant challenge to their commercial development. Although the radius of Zn2+ ions (0.74 Å) is almost the same as that of Li+ ions (0.76 Å), the larger atomic mass and stronger positive polarity result in poorer transport kinetics and lower solid-state solubility in bulk electrode. Thus, most electrode materials that can accommodate Li+ ions insertion/extraction are not suitable for ARZIBs. Only a few cathode materials have been demonstrated in a laboratory and most deliver limited specific capacities, poor rate capability and/or bad cycling performance.[3]